As shown in an example of FIG. 5, in a background art fuel cells system mounted on an electric vehicle, a reformer unit 128 receives supplies of fuel 124, for example methanol and water, fed via a pump 126 and produces a hydrogen-containing gaseous fuel from the fuel 124 through a steam reforming reaction of methanol. Fuel cells 136 receive a flow of the produced gaseous fuel and the air 130 and generate an electromotive force through electrochemical reactions of the gaseous fuel and the air 130. The electric power generated by the fuel cells 136 and the electric power output from a battery 140, which is connected in parallel with the fuel cells 136, are supplied to an inverter 144 to drive a motor 146 and obtain a driving force of the electric vehicle.
A control unit 120 calculates a required output (required electric power) of the inverter 144 from an accelerator travel of the electric vehicle measured by an accelerator pedal position sensor 122, and regulates the inverter 144 based on the calculated required output. Such regulation causes electric power corresponding to the required output to be supplied to the motor 146 via the inverter 144.
The fuel cells 136 output the electric power to cover the required output of the inverter 144. When the electric power output from the fuel cells 136 is insufficient for the required output, the battery 140 outputs the electric power to the inverter 144 to compensate for the insufficiency. The output electric power of the fuel cells 136 accordingly depends upon the required output of the inverter 144.
In response to a requirement of the output of electric power from the inverter 144, the fuel cells 136 can not output the required electric power in the case in which the gaseous fuel supplied from the reformer unit 128 to the fuel cells 136 is not sufficient for the output of the required electric power. That is, the output electric power of the fuel cells 136 also depends upon the quantity of the gaseous fuel (that is, the gas flow rate) fed to the fuel cells 136.
The control unit 120 drives the pump 126 based on the required output of the inverter 144, and regulates the quantities of the fuel 124 fed to the reformer unit 128, in order to regulate the quantity of the gaseous fuel supplied to the fuel cells 136 according to the required output of the inverter 144.
The quantity of the gaseous fuel produced by the reformer unit 128 does not immediately increase (or decrease) with an increase (or a decrease) in supplied quantities of the fuel 124, but increases or decreases after a time lag of 2 to 20 seconds. The quantity of the gaseous fuel required for the fuel cells 136 is thus not always identical with the actual supply of the gaseous fuel (the gas flow rate) to the fuel cells 136.
As described above, in the background art fuel cells system, the output electric power of the fuel cells 136 depends upon the required output of the inverter 144 and upon the quantity of the gaseous fuel (the gas flow rate) supplied to the fuel cells 136. The working point of the fuel cells 136 is thus varied with variations in required output of the inverter 144 and in gas flow rate.
FIG. 6 is a characteristic chart showing variations in power generation efficiency versus the output electric power in general fuel cells with a variation in quantity of the gaseous fuel (the gas flow rate) supplied to the fuel cells as a parameter. FIG. 7 is a characteristic chart showing a variation in output electric power versus the required quantity of the gaseous fuel in general fuel cells.
In the background art fuel cells system described above, as shown in FIG. 6, although the fuel cells are capable of being activated at a working point “a” of high power generation efficiency, the fuel cells may be activated, for example, at a working point “b” of low power generation efficiency since the actual working point is varied with a variation in gas flow rate.
In the background art fuel cells system described above, as shown in FIG. 7, even when a sufficient quantity Qc of the gaseous fuel is supplied from the reformer unit to the fuel cells to generate an output electric power Wc, the fuel cells may be activated, for example, at a working point “d” to generate only an output electric power Wd since the actual working point is varied with a variation in required output of the inverter. In this case, the quantity of the gaseous fuel required to generate the output electric power Wd is equal to only Qd, and the wasteful quantity of the gaseous fuel is (Qc−Qd). This lowers the utilization factor of the gaseous fuel.
As described above, in the background art fuel cells system, the working point of the fuel cells is varied with variations in required output of the inverter and in gas flow rate. The fuel cells are thus not always activated at the working point of high power generation efficiency or at the working point of high gas utilization factor.
The power generation efficiency and the gas utilization factor have a tradeoff relationship, so that it is difficult to enhance both the power generation efficiency and the gas utilization factor. Maximizing the product of the power generation efficiency and the gas utilization factor enhances both the power generation efficiency and the gas utilization factor as much as possible. The product of the power generation efficiency and the gas utilization factor is expressed as an energy conversion efficiency of the fuel cells.